The presently described and claimed inventive concept(s) relate to a method for producing electrolytes for electrochemical energy storage systems, and, more particularly, to a method for producing redox-active Ti(IV) coordination compounds in aqueous solution for use as electrolytes in flow battery systems.
A flow battery is a rechargeable fuel cell in which an electrolyte containing one or more dissolved electroactive elements flows through an electrochemical cell that reversibly converts chemical energy directly to electricity. Modern flow batteries are generally two electrolyte systems in which the two electrolytes, acting as liquid energy carriers, are pumped simultaneously through two half-cells separated by a membrane which comprise the reaction cell. On charging, supplied electrical energy causes a chemical reduction reaction in one electrolyte and an oxidation reaction in the other. A generally thin ion exchange membrane positioned between the half-cells prevents the electrolytes from mixing but allows selected ions to pass through to complete the redox reaction. On discharge the chemical energy contained in the electrolyte is released in the reverse reaction, and electrical energy can be drawn from the electrodes. When in use the electrolytes are continuously pumped in a circuit between reactor and storage tanks.
U.S. Pat. No. 8,753,761 B1 describes aqueous redox flow batteries which comprise metal ligand coordination compounds as a novel class of flow battery materials. Metal ligand coordination compounds, such as those comprising titanium, have been observed to exhibit high solubility, reversible electrochemistry (e.g., rapid electrochemical kinetics) and tunable redox potentials.
Production methods for Ti(IV) coordination compounds that can be used as electrolytes in flow batteries normally involve using precursors, such as, for example, TiCl4, titanium alkoxides, and the like as starting materials. These precursors are reacted with the corresponding complexing agents in water or a solvent. However, these precursor materials are all highly reactive and can be difficult to handle especially at large production scale. In addition, counter ions and by-products (e.g., chloride, alcohols, etc.) that are generated during the production process need to be separated and treated which tends to add significant cost to a commercial production process. Thus, the need exists for an improved more economical method for producing redox-active Ti(IV) coordination compounds of the type which are useful in electrochemical energy storage systems, and particularly in flow battery systems.